The corrosion problems of metal materials exist throughout various fields of domestic economy. Most metal materials have corrosion problems under acidic condition. The existing protecting measures for metal material corrosion mainly include improvement of structures and properties of materials themselves, plating, surface treatment such as painting, etc., control of ambient conditions where materials are used, cathode protection, usage of corrosion inhibitors, etc. Among these, corrosion inhibitors have various advantages of broad applicability, simple packaging process, low cost, long antirust period, good appearance, low energy consumption, etc., and has been widely used. However, the usage manner of one-time feeding or continual feeding is typically used in the conventional corrosion inhibitors. In initial period of use, the concentration of the corrosion inhibitor is very high, which may result in the waste of the corrosion inhibitor; as the time passes by, the corrosion inhibitor will encounter problems such as hydrolysis, degradation, deactivation, etc in service environment, which result in the reduction of corrosion inhibition effect. Therefore, in recent years, the researches have been devoted to develop an intelligent long-lasting sustained-release agent, which is desired to be capable of automatically controlling the release amount of a corrosion inhibitor according to the variation of the ambient environment. It is a direction of investigation and development of intelligent corrosion inhibitors to prepare an intelligent corrosion inhibitor by using a non-toxic and pH sensitive hydrogel as a carrier.
The sensitivity of the pH sensitive hydrogel comes from the ionizable groups, such as a carboxyl group, an amino group, etc., carried on the molecular chain thereof. These groups are subject to ionization or protonation under a certain pH condition and are positively or negatively charged. The mutual repelling between the same charges results in the change of swelling of hydrogel, represented by the pH sensitivity of hydrogel. As for a pH sensitive aqueous gel bearing a weakly basic group (typically an amino group), when the pH value of an ambient medium reaches to a specific value, the weakly basic group on the side chain of the molecular chain of the hydrogel is protonated to form an electrically charged cation. The repulsive force between the same kind of ions increases, and thereby the swelling of the hydrogel occurs. As for a pH sensitive hydrogel bearing a weakly acidic group, when the pH value of an ambient medium is greater than a specific value, the swelling of the hydrogel occurs.
The magnitude of swelling degree of the pH sensitive hydrogel under different ambient pH conditions is an important performance indication of this intelligent corrosion inhibitor. At a certain pH value, as the swelling degree of the hydrogel is larger, the releasing rate of the corrosion inhibiting substance is faster.
Since most metal materials have corrosion problems under acidic condition, it is very important to investigate an intelligent corrosion inhibitor, which can be released rapidly in a large amount under an acidic condition while can be released slowly in a small amount under a neutral or basic condition. Among various materials containing weakly basic groups, chitosan is a natural basic polysaccharide, which bears an amino group on C2 of the structural unit thereof, and is suitable for the preparation of the low-pH sensitive intelligent hydrogel required by intelligent corrosion inhibitors.